The invention relates to a turbocharger supercharged four-stroke internal combustion engine, in particular for a vehicle, this alternating piston engine comprising an intake manifold supplied in air under pressure by a compressor driven by a turbine itself supplied by a fraction at least of the exhaust gas of the engine, another fraction of these exhaust gases able to be recycled in the intake manifold in order to reduce the emissions of nitrogen oxides NOx.
The invention relates, more particularly, to an engine of the aforementioned type with two stages of turbocompression, comprising a low-pressure compressor and a high-pressure compressor mounted in series in order to supply the engine in compressed air, and driven respectively by a high-pressure turbine and by a low-pressure turbine, which are mounted in series and supplied by the exhaust gases of the engine.
In such an engine, the high-pressure turbine is dimensioned for the low engine rotating speeds, in order to improve the turbocompression at these speeds. A by-pass conduit equipped with an adjustable valve is connected between the intake and the outlet of the high-pressure turbine and makes it possible, for the highest rotating speeds of the engine, to discharge an excess flow of exhaust gas directly to the intake of the low-pressure turbine, which is dimensioned more generously.
However, this discharge of exhaust gas at the intake of the low-pressure turbine increases the pressure between the turbines and decreases the expansion ratio and the flow of the high-pressure turbine. This also increases the expansion ratio of the low-pressure turbine. Beyond a certain by-pass rate of the exhaust gases, the power provided by the high-pressure turbine is no longer sufficient to drive the compressor associated with this turbine.